The priority application No. 4-315759 filed in Japan on Oct. 30, 1992 is hereby incorporated hereinto by this reference.
1. Field of the Invention
The present invention relates to a fuel shut-off device which releases fuel vapor from a fuel tank and prevents fuel liquid from flowing out from the fuel tank.
2. Description of Related Art
Discharge of fuel liquid and fuel vapor from a fuel tank of a vehicle into the atmosphere is undesirable, for environmental protection. A fuel shut-off device is therefore provided with a fuel tank to prevent the discharge of the fuel. Another important function of the fuel shut-off device is to maintain the internal pressure of the fuel tank within a predetermined range, to thereby prevent the rupture of the fuel tank.
A conventional fuel shut-off device as disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 2-274622 comprises a float valve, which rises to shut-off preventing effusion of the fuel liquid from the fuel tank. More concretely, the float valve rises due to a flow of the fuel liquid into the fuel shut-off device to block a conduit in the device connected with the external apparatus.
The fuel shut-off device also comprises a positive pressure regulating valve which maintains the internal pressure of the fuel tank under a predetermined high level. The internal pressure of the fuel tank is regulated in the following manner. Suppose that the level of the fuel liquid is far below the fuel shut-off device. If the internal pressure of the fuel tank is increased by vaporization of the fuel liquid, the positive pressure regulating valve works to discharge the fuel vapor to an external apparatus, thereby mitigating the internal pressure of the fuel tank. The external apparatus is, for example, an activated charcoal canister which adsorbs fuel vapor.
The fuel shut-off device further comprises a negative pressure regulating valve which introduces fuel vapor from the external canister to the fuel tank to increase the internal pressure of the fuel tank when the internal pressure of the fuel tank is below a predetermined low level.
Now suppose that the float valve shuts-off to prevent leakage of the fuel liquid from the fuel tank. Since the positive pressure regulating valve is shut-off from the internal pressure of the fuel tank in this state, it cannot prevent the internal pressure of the fuel tank from increasing undesirably. The fuel shut-off device therefore comprises a safety valve for directly (not via the positive pressure regulating valve) releasing the fuel vapor from the tank to the canister to decrease the internal pressure of the fuel tank.
As described above, the conventional fuel shut-off device comprises the positive pressure regulating valve and the safety valve for decreasing the internal pressure of the fuel tank. Each of the positive pressure regulating valve and the safety valve includes a spring which generates a pressing force to close the valve. Since the opening pressure of the safety valve is greater than that of the positive pressure regulating valve, the safety valve has a stronger spring than that of the positive pressure regulating valve in the conventional device.